Analog controlled switch-mode power supplies (SMPS) are used in low power devices such as cell phones, portable data assistants, and MP3 players, to transfer variable supply (i.e. battery) voltage to a constant output value. The regulation is performed through the interaction of an analog controller and direct-current to direct-current (DC-DC) switching power converter. The task of the controller is to monitor the output voltage and provide appropriate low-power pulse-width modulated control signals for the switching converter, which efficiently processes power. To allow small weight and size of the overall system, it is usually desirable that the controller produces signals at a high constant frequency (switching frequency fsw) that does not interfere with the proper operation of the supplied device. In the existing portable devices dedicated analog integrated circuits (IC-s) are generally used to perform the control task. The analog controllers are fast and have low-power consumption but also suffer from many problems. They generally require a long and tedious design process and often need to be completely redesigned each time IC implementation technology changes, and as such are not flexible and unsuitable for the integration with fast changing digital hardware on which the majority of portable devices is based. In addition, the analog controllers are sensitive to temperature changes, variations in manufacturing process, and aging.
Digital control of low-power switch SMPS can result in significant improvements of system characteristics. It offers advantages such as simple introduction of advanced control laws and power management techniques, use of automated design tools that enable faster development and implementation, low sensitivity to external influences and parameter variations, realization with a small number of external passive components, and design portability, which allows simple transition from one to another, more advanced, IC implementation technology. All of these are highly desirable features in modern portable applications that need to be implemented with a miniature power-efficient hardware.
Modern tools for automatic digital design allow fast development process and simple transfer of designs from one implementation technology to another. In addition, the digital implementation benefits from superior flexibility. The same hardware can perform many different functions and existing designs can be easily altered to better suit other applications.
The digital hardware also shows lower susceptibility to external influences, such as change of temperature or aging. The operation of a digital system usually remains unchanged in all working conditions.
Moreover, the digital control also allows simple implementation of power savings (management) techniques based on voltage scaling that result in significant extension of the battery life. In those techniques, to allow minimal power consumption, the supply voltage of the device is changed in accordance with its processing load. Using digital hardware, these techniques can be implemented without a significant increase in system complexity. It can be done through simple communication with a digital microprocessor, which is a standard part of most modern portable devices. Implementation of the power savings techniques with analog hardware is a complex task. It requires additional hardware, and could increase power consumption and the size of the device.
Although the abovementioned advantages of the digital control are known, in low-power applications, analog pulse-width modulated (PWM) controllers are almost exclusively used. The main reasons for the sporadic use of digital controllers is their power consumption and much lower switching frequency compared to their analog counterparts. Power consumption of digital hardware is proportional to the product of switching frequency and size of the hardware (on-chip area) and in the existing solutions often exceeds the power consumed by the output load. As a result, overall efficiency of digitally controlled SMPS is poor. The lower switching frequency generally results in a larger, heavier, and more expensive power stage that can negate some or all of the abovementioned advantages of digital control.
The inferior performance of digitally controlled SMPS in low power applications is mainly caused by slow and power inefficient operation of basic functional blocks, digital pulse-width modulator, compensator and analog-to-digital controller. Recent products and publications (References 1-4 below) demonstrate digital controllers with improved performance. They allow introduction of digitally controlled SMPS in larger portable systems, such as laptop computers, and camcorders, but are still not suitable for smaller portable devices. For the targeted applications, they still have high power consumption and operate at relatively low switching frequencies. The maximum frequency of these solutions is between 400 kHz and 1 MHz, significantly lower than the switching frequency of readily available analog controllers that operate at frequencies up to 5 MHz (References 5-6 below). The known digital controllers will also not be able to operate with upcoming SMPS that, in near future, are expected to operate at switching frequencies significantly higher than 10 MHz.
Therefore what is needed is a device and method of digital control of low-power SMPS having low power consumption and being able to operate at switching frequencies even beyond 10 MHz (ultra-high switching frequency).